This invention relates generally to refrigerators, and more particularly to household refrigerators having a freezer compartment above a fresh food compartment.
Current household refrigerators have a cabinet construction that utilizes a relatively thin steel outer shell, together with a plastic inner liner with the space between being filled by an insulating polyurethane foam that is foamed in place to completely fill the insulation space. Generally, the inner liner is preferably formed from an acrylonitrile butadiene styrene copolymer, known as "ABS," because this plastic has desirable qualities of relatively high strength and hardness, together with excellent impact resistance, and its thermoplastic abilities lend themselves to thermoforming so that the entire liner may be fabricated from a single sheet of plastic. With this arrangement, the steel shell is generally formed to have a flat face at the front of the cabinet behind which the steel is formed into a re-entrant groove, while the liner has a flange on its front face which fits into this groove to provide a relatively close fit and eliminate the need for any separate thermal breaker strips between the liner and shell.
If the inner surface of the steel is untreated or unprotected, the polyurethane foam tends to adhere rather strongly to the steel surface, and likewise the foam will adhere quite strongly to an untreated surface of ABS plastic. This feature results in a strong structural sandwich construction, giving a high degree of rigidity to the entire cabinet. However, such strong bonding of the foam creates a problem because the plastic liner has a much greater thermal coefficient of expansion than that of the steel shell. When the cabinet is formed, both the shell and liner are at a relatively high temperature of about 55.degree. C., and after the cabinet cools, and particularly during shipping at low temperatures, the differential contractions between the shell and liner will produce stresses on the liner, which may then tend to crack and become damaged.
It has been recognize that some of the stress that is applied to the liner as a result of thermal expansion and contraction can be reduced by the application of release agents, such as waxes, or certain plastic material, such as polyethylene and polyvinyl acetate, that are non-adherent to the polyurethane foam. These materials are selectively applied to areas on both the liner and the outer shell, particularly at places of maximum stress, and the use of these coatings has generally reduced the problem. In addition, advantage is taken of the formability of the thermoplastic liner by forming inwardly projecting ribs or ridges on the liner to accommodate shelves and also to position the partition between the freezer compartment and lower food compartment. The presence of these ribs, particularly if coated with a release agent to prevent strong adherence of the polyurethane foam, allows the plastic liner to expand and contract vertically along what may be called "accordion" lines, which tends to greatly reduce the stress on the liner. Recognition of the above approaches to reduce liner stress is shown, for example, in U.S. Pat. Nos. 3,813,137; 3,858,409; 3,940,195; and 4,498,713.
However, it has been found that, while the above approaches do tend to reduce cracking throughout most of the liner, they are not effective to reduce the stresses on the liner around the front flange that fits into the groove in the outer shell. While it is noted that U.S. Pat. No. 4,191,434 shows a construction in which the front flange is recessed in a rectangular notch to receive a flange on the partition between the freezer and food compartments, such an arrangement is difficult to form and still results in sharp corners which tend to concentrate the thermal stress and, therefore, do not eliminate the cracking at the front flange.
One arrangement that does recognize this problem and provide a solution is shown in U.S. Pat. No. 4,771,532, in which an elongated slot is cut through the front flange and a portion of the side wall of the liner. While this arrangement does provide a certain amount of mechanical compliance for the liner for reduce thermal stress in this area, it is relatively expensive to form, since the liner must be cut away after forming, and the slot must be covered during the foaming operation to prevent the foam from entering into the interior of the liner.